The invention relates to a method and a system for congestion management in a frame relay network.
Congestion means a situation in which the number of transmission requests exceeds the transmission capacity at a certain network point (called a bottle-neck resource) at a specific time. Congestion usually results in overload conditions, as a result of which the buffers overflow, for instance, and so packets will be retransmitted either by the network or the subscriber. The function of congestion management (CM) is to maintain a balance between the transmission requests and the transmission capacity so that the bottle-neck resources operate on an optimal level, and the subscribers are offered service in a way that assures fairness.
Congestion management can be divided into congestion avoidance (CA) and congestion recovery (CR). Congestion avoidance methods aim at preventing the occurrence of congestion in the network by dynamically adjusting the bandwidth of the subscribers in accordance with network congestion conditions and/or by altering the network routing so that part of the traffic load of the bottle-neck resources is shifted to idle resources. The purpose of recovery methods, in turn, is to restore the operation of the bottle-neck resources to an optimal level if the avoidance methods have failed to prevent the occurrence of congestion.
The frame relay (FR) technique is a packet-switched network technique used for the transmission of frames of varying length in place of the packet-switched network connections earlier placed in use. The protocol (X.25) applied generally in the present packet-switched networks requires plenty of processing and the transmission equipment is expensive, as a result of which the speeds also remain low. These matters are due to the fact that the X.25 standard was developed when the transmission connections used were still rather prone to transmission errors. The starting point of the frame relay technique was a considerably lower transmission line error probability. It has therefore been possible to discard a number of unnecessary functions in the frame relay technique, which makes the delivery of frames rapid and efficient. The Frame Mode Bearer Service is described generally in CCITT recommendation I.233 (Reference 1) and the associated protocol in recommendation Q.922 (Reference 2). Congestion in an FR network and congestion management mechanisms are described in CCITT recommendation I.370 (Reference 3). For a more detailed description of the FR technique, reference is made to An Overview of Frame Relay Technology, Datapro Management of Data Communications, McGraw-Hill Incorporated, April 1991, (Reference 4) as well as the above-mentioned recommendations.
In the FR network hierarchies presently in use, nodes have reception and transmission buffers corresponding only to the physical channels, i.e. the traffic of several different virtual channels and applications passes through the same buffer. Buffers are emptied as far as possible to links outbound from the node irrespective of the total level of congestion in the network. Links outbound from the node are thus loaded as much as possible even if the frame will probably be discarded closer to the middle of the network (at a congested node). In addition to wasting the network resources, discarding of frames at trunk nodes of the network affects the applications using the network in the form of longer throughput delays. (The virtual channel refers to a virtual connection portion having the length of one transmission link while the virtual connection is the actual packet-switched end-to-end FR connection.)
Even though the traffic received from virtual channels is monitored on subscriber connections on the basis of general service parameters Bc, Be and CIR, the monitoring is not effected when frames are forwarded from the subscriber node. (Parameter Bc (committed burst size) represents the maximum amount of data the subscriber can transmit over the network within a certain time slot Tc (usually Tc=1s); parameter Be (excess burst size) represents the amount of data by which the subscriber can exceed the value Bc within the time slot Tc; and parameter CIR (committed information rate) represents the data transmission rate guaranteed by the network under normal conditions, CIR=Bc/Tc. These parameters are defined in Reference 3.) From the reception buffer, the frame is routed to the correct channel-specific transmission buffer. Frames arriving at the node thus pass through the entire node on the FIFO principle. Due to the burstiness of the traffic in a frame relay network, it often follows from the above that one virtual channel takes a major part of the buffering and relay capacity of a node. This affects the other virtual channels supplying traffic to the node both as longer throughput delays and as higher frame loss probabilities. Even one virtual channel with a highly bursty or unsatisfactory nature may cause the other virtual channels of the connection to be subjected to a considerable fall in the service level.